Seen by many as the group which links their romantic precursors with their surrealist successors, the fin-de-siècle French poets who critics call symbolists were undeniably influential. Their structures and conceits are built upon grand, illogical, intuitive associations. The "symbols" for which they are named are emblems of the actual world--as opposed to the purely emotional world which dominates their work--that accumulate supernatural significance in the absence of a clear narrative or location.
Charles Baudelaire is perhaps the most influential of the symbolists. His monumental collection Les fleurs du mal (Flowers of Evil) was published in 1857. The book's dark, introspective, multifarious worlds revealed subjects and styles that had been previously barred from poetic inclusion, and for this reason Baudelaire was alternately celebrated and condemned as a heretical and even obscene innovator.
Stéphane Mallarmé, Paul Verlaine, and Arthur Rimbaud were followers and friends of Baudelaire, and Mallarmé termed their group The Decadents. Indeed, they lived tumultuous, bohemian lives, and the affair between Verlaine and Rimbaud has been the source of many legends and dramatizations.
Of the group, Verlaine received the most recognition for his work during his lifetime and, despite his erratic and even criminal behavior, he was elected "Prince of Poets" by the review La Plume shortly before his death. Though he strove to make his poetry one of intensity and extremity, his form was almost classical in its control and musicality.
Rimbaud is a rare poetic figure in that he stopped writing at the age of 21. His work, however, remains a passionate, visionary body that is continually and widely read. Though they received little public recognition until after his death, Une saison en enfer (A Season in Hell) and Illuminations became models for poets striving to make visible the tormented soul.
Though Mallarmé’s work was initially met with hostility for its difficulty and obscurity, his experimental work and his intricate theories eventually made him a favorite for twentieth-century writers and readers. Thanks to his disavowal of tradition, his unconventional syntax, his indirect expression, and his resistance to criticism, adventurous writers continue to extend his methods into contemporary poetics.
Paul Valéry is also considered one of the most important symbolist thinkers, though he might be more accurately described as a pivotal figure between many poetic schools. His work built upon the work of the writers mentioned above, and eventually became a foundation for twentieth-century modernists and structuralists.
For a general overview & more links, visit the symbolism resources at Poetry Portal.